Knitting jumper Help!

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I knit a lot, but tend to make up my own patterns for hats, socks, toys etc.

I’ve bought some beautiful wool and can visualise the jumper I want to make, but not sure about the neckline. Is there a website where you can insert the size from a swatch and it’ll tell you how to make it/when to decrease for sleeves etc?

I don't know of any websites but both Elizabeth Zimmerman and Cocoknits Workshop construct patterns a bit like this I.e using a formula rather than a pattern. I think they are hard copy books only though.

Thanks. In the end I’ve found a jumper I own and am attempting it by copying that...